Calculation and Comparison of Electronic, Vibrational, Polarization, and Magnetic Properties of Double Perovskites CaMnTi2O6 and CaFeTi2O6

Vibrational, polarization, magnetic, and electronic properties of double perovskites CaMnTi2O6 and CaFeTi2O6 with a rare type of "column" ordering of divalent metal cations have been calculated based on the density functional theory. Analysis of the crystal lattice dynamics for paraelectric phase P4(2)/nmc of both compounds has revealed that ferroelectric instability exists only in CaMnTi2O6. It is found that the structure distortion of the paraphrase of CaMnTi2O6 in the eigenvector of the unstable polar mode leads to a structure with the P4(2)/nmc space group. The calculated spontaneous polarization for the ferroelectric phase of CaMnTi2O6 is P-s = 25 mu C/cm(2). The spin-polarization calculations have shown that the ground state is ferromagnetic in the CaFeTi2O6 crystal and antiferromagnetic in the CaMnTi2O6 crystal. The exchange interaction constants have been calculated using the Heisenberg model and the mean field approximation; the phase transition temperature for each compound has been estimated.